The invention relates to a circuit configuration for controlling a pump current of an exhaust probe in a motor vehicle.
As environmental awareness is increasing and resulting exhaust gas regulations are becoming increasingly strict, the need to reduce pollutants in exhaust gases of internal combustion engines in motor vehicles is becoming increasingly important. Compliance with currently valid emission limits for pollutants such as carbon monoxide (CO), nitrogen oxides (NOx) and hydrocarbons (HC) requires, on the one hand, a specific engine control and, on the other hand, a catalytic post-treatment of the exhaust gases. For both measures it is necessary to obtain measurement values with exhaust gas probesxe2x80x94for example lambda probes or NOx probes. In the following, the term probe designates a unit including a sensor, a sensor line and a sensor plug.
It is known to use thick film sensors to measure the concentration of pollutants in the exhaust gas of an internal combustion engine. Such a sensor is described, using the example of a NOx sensor, by N. Kato et al. in the publication xe2x80x9cPerformance of Thick Film NOx Sensor on Diesel and Gasoline Enginesxe2x80x9d, Society of Automotive Engineers, Publication 970858, 1997. This NOx sensor has two measuring cells and three oxygen pump cells and implements the following measuring concept: in a first measuring cell to which the gas to be measured is fed via a diffusion barrier, a first oxygen concentration is set through the use of a first oxygen ion pump current, wherein no NOx is decomposed. In a second measuring cell, which is connected to the first measuring cell via a diffusion barrier, the oxygen content is reduced further through the use of a second oxygen ion pump current and NOx is decomposed at a measuring electrode. The oxygen which is generated in this way is used as a measure of the NOx concentration. The entire NOx sensor is heated to an increased temperature, for example 700xc2x0 C., through the use of an electric heating element.
In order to operate such a sensor it is necessary to regulate the respective pump current for the oxygen pump cells precisely.
It is accordingly an object of the invention to provide a circuit configuration which can control the pump currents for the oxygen pump cells in a precise manner.
With the foregoing and other objects in view there is provided, in accordance with the invention, in combination with an exhaust probe having a solid electrolyte, a pump cell and a measuring cell, and operating according to a principle of a galvanic oxygen concentration cell, a circuit configuration for controlling a pump current for the pump cell, including:
a microcontroller for determining a pump current actual value at the pump cell to be controlled, the microcontroller determining an actual value of a Nernst voltage at the measuring cell, and the microcontroller generating a pulse-width-modulated signal;
an analog circuit connected to the microcontroller, the analog circuit converting the pulse-width-modulated signal into the pump current for the pump cell; and
a read-only memory operatively connected to the microcontroller, the read-only memory providing a setpoint value of the Nernst voltage.
According to a preferred embodiment of the invention, a microcontroller in conjunction with an analog circuit is used to control the required pump currents. In order to determine the respective pump current, the voltage drop across a measuring resistor which is preferably connected directly upstream of the corresponding pump cell is measured using A/D converters in the microcontroller. In this way, the measuring error of the entire circuit configuration is reduced to the tolerance of the measuring resistors and the errors of the A/D conversions caused by leakage currents and quantization errors.
According to another feature of the invention, the read-only memory is a programmable read-only memory.
According to yet another feature of the invention, the read-only memory is integrated into the microcontroller.
According to a further feature of the invention, a measuring resistor is connected upstream of the pump cell to be controlled, and the microcontroller determines the pump current actual value from a voltage drop across the measuring resistor.
According to another feature of the invention, the microcontroller includes a first difference former, a second difference former, a first controller, a second controller, and a pulse-width modulation unit. The first difference former forms a voltage difference from the actual value of the Nernst voltage and the setpoint value of the Nernst voltage. The first controller forms a pump current setpoint value from the voltage difference. The second difference former forms a pump current difference value from the pump current actual value and the pump current setpoint value. The pulse-width modulation unit generates the pulse-width-modulated signal, and the second controller controls the pulse-width modulation unit based on the pump current difference value.
According to another feature of the invention, the microcontroller includes an first A/D converter and a third difference former. A measuring resistor is connected upstream of the pump cell to be controlled. The first A/D converter reads voltage potentials upstream and downstream of the measuring resistor, and the third difference former determines the pump current actual value from the voltage potentials read by the A/D converter.
According to a further feature of the invention, the microcontroller includes a second A/D converter and a fourth difference former. The second A/D converter reads a Nernst potential and a reference potential at the measuring cell, and the fourth difference former determines the actual value of the Nernst voltage from the Nernst potential and the reference potential.
According to another feature of the invention, the analog circuit includes a filter circuit and an impedance converter. The filter circuit converts the pulse-width-modulated signal into a DC voltage signal, and the impedance converter adapts an output impedance of the microcontroller as a function of a required current strength.
According to yet another feature of the invention, a sensor plug housing for the exhaust probe is provided, and the microcontroller, the analog circuit, and the read-only memory are integrated in the sensor plug housing.
With the objects of the invention in view there is also provided, an exhaust probe configuration, including:
an exhaust probe including a solid electrolyte, a pump cell, and a measuring cell, the exhaust probe operating according to a principle of a galvanic oxygen concentration cell;
a circuit configuration connected to the exhaust probe for controlling a pump current for the pump cell;
the circuit configuration including a microcontroller, an analog circuit, and a read-only memory;
the microcontroller determining a pump current actual value at the pump cell to be controlled, the microcontroller determining an actual value of a Nernst voltage at the measuring cell, and the microcontroller generating a pulse-width-modulated signal;
the analog circuit being connected to the microcontroller, the analog circuit converting the pulse-width-modulated signal into the pump current for the pump cell; and
the read-only memory providing a setpoint value of the Nernst voltage.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit configuration for controlling a pump current of an exhaust probe in a motor vehicle, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.